Various approaches to automated or semi-automated three-dimensional object production or Rapid Prototyping & Manufacturing have become available in recent years, characterized in that each proceeds by building up 3D objects from 3D computer data descriptive of the objects in an additive manner from a plurality of formed and adhered laminae. These laminae are sometimes called object cross-sections, layers of structure, object layers, layers of the object, or simply layers (if the context makes it clear that solidified structure of appropriate shape is being referred to). Each lamina represents a cross-section of the three-dimensional object. Typically lamina are formed and adhered to a stack of previously formed and adhered laminae. In some RP&M technologies, techniques have been proposed which deviate from a strict layer-by-layer build up process wherein only a portion of an initial lamina is formed and prior to the formation of the remaining portion(s) of the initial lamina, at least one subsequent lamina is at least partially formed.
According to one such approach, a three-dimensional object is built up by applying successive layers of unsolidified, flowable material to a working surface, and then selectively exposing the layers to synergistic stimulation in desired patterns, causing the layers to selectively harden into object laminae which adhere to previously-formed object laminae. In this approach, material is applied to the working surface both to areas which will not become part of an object lamina, and to areas which will become part of an object lamina. Typical of this approach is Stereolithography (SL), as described in U.S. Pat. No. 4,575,330, to Hull. According to one embodiment of Stereolithography, the synergistic stimulation is radiation from a UV laser, and the material is a photopolymer. Another example of this approach is Selective Laser Sintering (SLS), as described in U.S. Pat. No. 4,863,538, to Deckard, in which the synergistic stimulation is IR radiation from a CO.sub.2 laser and the material is a sinterable powder. This first approach may be termed photo-based stereolithography. A third example is Three-Dimensional Printing (3DP) and Direct Shell Production Casting (DSPC), as described in U.S. Pat. Nos. 5,340,656 and 5,204,055 , to Sachs, et al., in which the synergistic stimulation is a chemical binder (e.g. an adhesive), and the material is a powder consisting of particles which bind together upon selective application of the chemical binder.
According to a second such approach, an object is formed by successively cutting object cross-sections having desired shapes and sizes out of sheets of material to form object lamina. Typically in practice, the sheets of paper are stacked and adhered to previously cut sheets prior to their being cut, but cutting prior to stacking and adhesion is possible. Typical of this approach is Laminated Object Manufacturing (LOM), as described in U.S. Pat. No. 4,752,352, to Feygin in which the material is paper, and the means for cutting the sheets into the desired shapes and sizes is a CO.sub.2 laser. U.S. Pat. No. 5,015,312 to Kinzie also addresses building object with LOM techniques.
According to a third such approach, object laminae are formed by selectively depositing an unsolidified, flowable material onto a working surface in desired patterns in areas which will become part of an object laminae. After or during selective deposition, the selectively deposited material is solidified to form a subsequent object lamina which is adhered to the previously-formed and stacked object laminae. These steps are then repeated to successively build up the object lamina-by-lamina. This object formation technique may be generically called Selective Deposition Modeling (SDM). The main difference between this approach and the first approach is that the material is deposited only in those areas which will become part of an object lamina. Typical of this approach is Fused Deposition Modeling (FDM), as described in U.S. Pat. Nos. 5,121,329 and 5,340,433, to Crump, in which the material is dispensed in a flowable state into an environment which is at a temperature below the flowable temperature of the material, and which then hardens after being allowed to cool. A second example is the technology described in U.S. Pat. No. 5,260,009, to Penn. A third example is Ballistic Particle Manufacturing (BPM), as described in U.S. Pat. Nos. 4,665,492; 5,134,569; and 5,216,616, to Masters, in which particles are directed to specific locations to form object cross-sections. A fourth example is Thermal Stereolithography (TSL) as described in U.S. Pat. No. 5,141,680, to Almquist et. al.
When using SDM (as well as other RP&M building techniques), the appropriateness of various methods and apparatus for production of useful objects depends on a number of factors. As these factors cannot typically be optimized simultaneously, a selection of an appropriate building technique and associated method and apparatus involve trade offs depending on specific needs and circumstances. Some factors to be considered may include 1) equipment cost, 2) operation cost, 3) production speed, 4) object accuracy, 5) object surface finish, 6) material properties of formed objects, 7) anticipated use of objects, 8) availability of secondary processes for obtaining different material properties, 9) ease of use and operator constraints, 10) required or desired operation environment, 11) safety, and 12) post processing time and effort.
In this regard there has been a long existing need to simultaneously optimize as many of these parameters as possible to more effectively build three-dimensional objects. As a first example, there has been a need to enhance object production speed when building objects using the third approach, SDM, as described above (e.g. Thermal Stereolithography) while simultaneously maintaining or reducing the equipment cost. As a second example, there has been a long existing need for a low cost RP&M system useable in an office environment.
In SDM, as well as the other RP&M approaches, typically accurate formation and placement of working surfaces are required so that outward facing cross-sectional regions can be accurately formed and placed. The first two approaches naturally supply working surfaces on which subsequent layers of material can be placed and lamina formed. However, since the third approach, SDM, does not necessarily supply a working surface, it suffers from a particularly acute problem of accurately forming and placing subsequent lamina which contain regions not fully supported by previously dispensed material such as regions including outward facing surfaces of the object in the direction of the previously dispensed material. In the typical building process where subsequent laminae are placed above previously formed laminae this is particularly a problem for down-facing surfaces (down-facing portions of laminae) of the object. This can be understood by considering that the third approach theoretically only deposits material in those areas of the working surface which will become part of the corresponding object lamina. Thus, nothing will be available to provide a working surface for or to support any down-facing surfaces appearing on a subsequent cross-section. Downward facing regions, as well as upward facing and continuing cross-sectional regions, as related to photo-based Stereolithography, but as applicable to other RP&M technologies including SDM, are described in detail in U.S. Pat. Nos. 5,345,391, and 5,321,622, to Hull et. al. and Snead et. al., respectively. The previous lamina is non-existent in down-facing regions and is thus unavailable to perform the desired support function. Similarly, unsolidified material is not available to perform the support function since, by definition, in the third approach, such material is typically not deposited in areas which do not become part of an object cross-section. The problem resulting from this situation may be referred to as the "lack of working surface" problem.
The "lack of working surface" problem is illustrated in FIG. 1, which depicts two laminae, identified with numerals 1 and 2, built using a three-dimensional modeling method and apparatus. As shown, lamina 1, which is situated on top of lamina 2, has two down-facing surfaces, which are shown with cross-hatch and identified with numerals 3 and 4. Employing the SDM approach described above, unsolidified material is never deposited in the volumes directly below the down-facing surfaces, which are identified with numerals 5 and 6. Thus, with the SDM approach, there is nothing to provide a working surface for or to support the two down-facing surfaces.
Several mechanisms have been proposed to address this problem, but heretofore, none has proven completely satisfactory. One such mechanism, suggested or described in U.S. Pat. No. 4,247,508, to Housholder; U.S. Pat. Nos. 4,961,154; 5,031,120; 5,263,130; and 5,386,500, to Pomerantz, et al.; U.S. Pat. No. 5,136,515, to Helinski; U.S. Pat. No. 5,141,680, to Almquist, et al.; U.S. Pat. No. 5,260,009, to Penn; U.S. Pat. No. 5,287,435, to Cohen, et al., U.S. Pat. No. 5,362,427, to Mitchell; U.S. Pat. No. 5,398,193, to Dunghills; U.S. Pat. Nos. 5,286,573 and 5,301,415, to Prinz, et al., involves filling the volumes below down-facing surfaces with a support material different from that used to build the object, and presumably easily separable from it (by means of having a lower melting point, for example). In relation to FIG. 1, for example, the volumes identified with numerals 5 and 6 would be filled with the support material prior to the time that the material used to form down-facing surfaces 3 and 4 is deposited.
A problem with the two material (i.e. building material and different support material) approach is that it is expensive and cumbersome because of the inefficiencies, heat dissipation requirements, and costs associated with handling and delivering the support, or second, material. For example, a separate material handling and dispensing mechanism for the support material may have to be provided. Alternatively, means may have to be provided to coordinate the handling and delivery of both materials through a single system.
Another approach, described in U.S. Pat. No. 4,999,143, to Hull, et al.; U.S. Pat. No. 5,216,616, to Masters; and U.S. Pat. No. 5,386,500, to Pomerantz, et al., is to build generally spaced support structures from the same material as that used to build the object. A multitude of problems have occurred with this approach. A first problem has involved the inability to make support structures of arbitrary height while simultaneously ensuring that they were easily separately from the object. Second, a problem has been encountered regarding the inability to achieve easy separability between object and support structure while simultaneously maintaining an effective working surface for the building of and support of the outward facing surfaces. A third problem involves the inability to accumulate support structure in the direction perpendicular to the planes of the cross-sections (e.g. vertical direction) at approximately the same rate as that at which the object accumulates. A fourth problem has involved the inability to ensure easy separability and minimal damage to up-facing surfaces when supports must be placed thereon in order to support down-facing surfaces thereabove which are part of subsequent layers. A fifth issue has involved the desire to increase system throughput.
To illustrate, the objective of achieving easy separability dictates that the surface area over which each support contacts the object be kept as small as possible. On the other hand, the objective of accumulating a support in the Z-direction at a rate approximating that of object accumulation dictates that the cross-sectional area of each support be as large as possible to provide a large area to perimeter ratio thereby minimizing loss of material for build up in the Z-direction due to run off, spreading, mis-targeting and the like by allowing a large target area to compensate for any inaccuracies in the deposition process and to limit the ability of material to spread horizontally instead of building up vertically.
Further, the objective of achieving minimal damage to the down-facing surface dictates that the spacing between the supports be kept as large as possible in order to minimize the area of contact between the supports and the object. On the other hand, the objective of providing an effective working surface for the building of the down-facing surface dictates that the spacing be kept as small as possible. As is apparent, there is a conflict in simultaneously achieving these objectives.
This problem is illustrated in FIG. 2, in which, compared to FIG. 1, like elements are referenced with like numerals. As shown, down-facing surface 3 is supported through columnar supports 7a, 7b, and 7c, while down-facing surface 4 is supported through columnar supports 8a, 8b, 8c, and 8d. Columnar supports 7a, 7b, and 7c are widely spaced from one another in order to minimize damage to down-facing surface 3. Moreover, they are each configured to contact the down-facing surface over a relatively small surface area to enhance separability. On the other hand, because of their small cross-sectional surface area, they may not be able to accumulate, in the vertical direction, fast enough to keep up with the rate of growth of the object. Moreover, because of their wide spacing, they may not be able to provide an effective working surface for the building of and support of down-facing surface 3.
Columnar supports 8a, 8b, 8c, and 8d, by contrast, are more closely spaced together in order to provide a more effective working surface for the building and support of down-facing surface 4. Also, each is configured with a larger surface area to enable them to grow at rate approximating that of the object. Unfortunately, because of their closer spacing and larger cross-sectional area, these supports will cause more damage to the down-facing surface when they are removed.
All patents referred to herein above in this section of the specification are hereby incorporated by reference as if set forth in full.